Messages in this thread |  | | Subject | Re: [PATCH] block: BFQ default for single queue devices | From | Bart Van Assche <> | Date | Sat, 6 Oct 2018 09:20:36 -0700 |
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On 10/5/18 11:46 PM, Paolo Valente wrote: >> Il giorno 06 ott 2018, alle ore 05:12, Bart Van Assche <bvanassche@acm.org> ha scritto: >> On 10/5/18 2:16 AM, Jan Kara wrote: >>> On Thu 04-10-18 15:42:52, Bart Van Assche wrote: >>>> What I think is missing is measurement results for BFQ on a system with >>>> multiple CPU sockets and against a fast storage medium. Eliminating >>>> the host lock from the SCSI core yielded a significant performance >>>> improvement for such storage devices. Since the BFQ scheduler locks and >>>> unlocks bfqd->lock for every dispatch operation it is very likely that BFQ >>>> will slow down I/O for fast storage devices, even if their driver only >>>> creates a single hardware queue. >>> Well, I'm not sure why that is missing. I don't think anyone proposed to >>> default to BFQ for such setup? Neither was anyone claiming that BFQ is >>> better in such situation... The proposal has been: Default to BFQ for slow >>> storage, leave it to deadline-mq otherwise. >> >> How do you define slow storage? The proposal at the start of this thread >> was to make BFQ the default for all block devices that create a single >> hardware queue. That includes all SATA storage since scsi-mq only creates >> a single hardware queue when using the SATA protocol. The proposal to make >> BFQ the default for systems with a single hard disk probably makes sense >> but I am not sure that making BFQ the default for systems equipped with >> one or more (SATA) SSDs is also a good idea. Especially for multi-socket >> systems since BFQ reintroduces a queue-wide lock. > > No, BFQ has no queue-wide lock. The very first change made to BFQ for > porting it to blk-mq was to remove the queue lock. Guided by Jens, I > replaced that lock with the exact, same scheduler lock used in > mq-deadline.
It's easy to see that both mq-deadline and BFQ define a queue-wide lock. For mq-deadline its deadline_data.lock. For BFQ it's bfq_data.lock. That last lock serializes all bfq_dispatch_request() calls and hence reduces concurrency while processing I/O requests. From bfq_dispatch_request():
static struct request *bfq_dispatch_request(struct blk_mq_hw_ctx *hctx) { struct bfq_data *bfqd = hctx->queue->elevator->elevator_data; [ ... ] spin_lock_irq(&bfqd->lock); [ ... ] }
I think the above makes it very clear that bfqd->lock is queue-wide.
It is easy to understand why both I/O schedulers need a queue-wide lock: the only way to avoid race conditions when considering all pending I/O requests for scheduling decisions is to use a lock that covers all pending requests and hence that is queue-wide.
Bart.
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